DOC-2487: Replace 'en' with 'en_US' in Spellchecker docs and demo

[FarzadHayat]

Ticket: DOC-2487 6 Docs: https://github.com/tinymce/tinymce-docs/pull/3393

Site: Spellchecker demo (see JS tab) Site: Spellchecker docs

Search terms used to find any occurences of 'en':

• spellchecker_language:
• spellchecker_languages:
• 'en'

Changes:

• Replace 'en' with 'en_US' in Spellchecker docs and demo

Pre-checks:

• [x] Branch prefixed with feature/<version>/, hotfix/<version>/, staging/<version>/, or release/<version>/.
• [-] modules/ROOT/nav.adoc has been updated (if applicable).
• [-] Included a release note entry for any New product features.
• [-] If this is a minor release, updated productminorversion in antora.yml and added new supported versions entry in modules/ROOT/partials/misc/supported-versions.adoc.

Review:

• [x] Documentation Team Lead has reviewed

[kemister85]

We might need to update this table to remove en reference.